char schar

(char string index) -- (schar string index)

Returns the character at the 0-based index of string. char and schar behave identically here; in Common Lisp schar is the simple-string variant, but rontolisp treats them the same. The WASM backend indexes strings by byte, so indexing is well-defined for ASCII text only. A string that is not a string or an index that is not an integer signals a type-error naming the operator (CHAR: The value 5 is not of type STRING); the setf forms below report as (SETF CHAR) / (SETF SCHAR), and so does a stored value that is not a character ((SETF CHAR): The value 5 is not of type CHARACTER).

Both are also setf places: (setf (schar s i) c) / (setf (char s i) c) replaces the character at i and returns c. A string the running program allocated -- a make-string buffer, say -- is mutated in place. A string literal is never mutated on any backend: it is the constant in your source, shared by every evaluation of the form it appears in, so the write rebuilds the string and rebinds the place instead. That means the string expression must be a variable when it holds a literal (writing through (setf (char "abc" 0) #\Z), or through a literal reached by an accessor, is an error), and an alias made before the write still sees the literal's own content. On the compiled backends a string built by copy-seq/subseq, concatenate 'string, the string-upcase family, format nil, with-output-to-string or read-line is mutable like a make-string buffer, so aliases of it see the write; the rebuild-and-rebind also applies to the few producers whose results are still immutable values there (princ-to-string, for example), where an alias never sees such a write.